From the Company Login screen, choose the Company you would like to be your default and mark the box for Remember this company.

Remember this company

Mark to log in to the selected company automatically without using the Company Login window. You must restart Microsoft Dynamics GP before this option takes effect. You should mark this option for the company you work with the most often.

If you mark this option and restart Microsoft Dynamics GP, you can switch companies or unmark this option by opening the Company Login window.